1. Field of the Invention
The present invention relates to a developer composition suitable for developing an electrostatically charged image in electrophotography, electrostatic recording, electrostatic printing and the like.
2. Discussion of Related Art
As described in U.S. Pat. Nos. 2297691 and 2357809, the electrophotography of the prior art comprises electrically charging a photoconductive insulating layer uniformly, partially discharging the layer by exposure to light to form an electrically charged latent image, making an electrically charged color fine powder (a so-called toner) adhere to the latent image (developing step), transferring the obtained visible image to a transfer material such as transfer paper (transfer step) and permanently fixing the transferred image by a suitable fixing method such as heating and pressing.
Accordingly, a toner must satisfy the functions required not only in the developing step but also in the transfer and fixing steps.
Generally, a toner undergoes mechanical friction caused by shear and shock, while it behaves mechanically in a developing device. Therefore, it is deteriorated by repeating duplication several thousands to several tens of thousands times. Such deterioration of a toner can be inhibited by the use of a resin having a high molecular weight which is tough enough to stand the mechanical friction. However, since such a resin generally has a high softening point, a toner containing such a resin can not be fixed sufficiently firmly by a non-contact fixing method of a low thermal efficiency, such as oven fixing or radiant fixing using infrared rays. Even when such a toner is fixed by a heat roller fixing method which is a contact method excellent in thermal efficiency, the roller must be heated to a considerably high temperature in order to attain sufficiently firm fixing and the fixing at such a high temperature causes deterioration of a fixing device, curling of paper, increase in energy consumption and the like. Further, the production efficiency of a toner containing such a high-molecular weight resin is remarkably low, because such a resin is not readily pulverized. Thus, it is unsuitable to use a resin having a high degree of polymerization and a high softening point as a binder of a toner. On the other hand, the heat roller fixing method involves contacting the surface of a heated roller with the surface of a toner image, so that it is remarkably excellent in thermal efficiency to be widely employed not only in high-speed fixing but also in low-speed fixing. However, the heat roller fixing method is disadvantageous in that the toner constituting a transferred image tends to adhere to a heated roller, when the surface of the toner image is contacted with the roller, and that the toner thus adhering to the heated roller tends to be retransferred to the succeeding transfer paper. That is, the heat roller fixing method tends to cause a so-called offset phenomenon. In order to inhibit this phenomenon, the surface of the roller is processed with a material excellent in release properties, such as a fluorinated resin, and further coated with a release agent such as a silicone oil.
However, this method of applying silicone oil requires an enlarged fixing device to therefore bring about not only an increase in cost but also complicatedness, thus being unfavorable.
A process for the inhibition of offset phenomenon which comprises using a resin having a wide molecular weight distribution as a binder resin is known as described in Japanese Patent Publication No. 6895/1980 and Japanese Patent Laid-Open No. 98202/1981. However, such a resin generally has a high degree of polymerization and therefore requires a high fixing temperature.
Further, a process which comprises unsymmetrizing or crosslinking a binder resin to inhibit offset has been reported as a more effective process in, for example, Japanese Patent Publication No. 493/1982 and Japanese Patent Laid-Open Nos. 44836/1975 and 37353/1982. However, this process does not improve the fixability.
Since the lowest fixing temperature is generally between a low-temperature offset disappearing temperature and a high-temperature offset initiating temperature, the operating temperature range is between the minimum fixing temperature and the high temperature offset-initiating temperature. Therefore, the practical fixing temperature can be lowered with an enlargement of the operable temperature range by lowering the lowest fixing temperature as low as possible and enhancing the high-temperature offset initiating temperature as high as possible to thereby attain energy reduction, high-speed fixing and inhibition of paper curling. Further, lowering to the lowest fixing temperature and enhancement of the high-temperature offset initiating temperature allow copying on both sides without causing any trouble to bring about various advantages such as rendering a duplicating machine intelligent and relaxation of accuracy and tolerance of temperature control of a fixing device.
Thus, development of a resin excellent in fixability and offset resistance and a toner containing such a resin has long been expected.
It is known as described in Japanese Patent Laid-Open Nos. 65232/1974, 28840/1975 and 81342/1975 that the offset resistance of a toner containing a styrene resin as a binder can be enhanced by adding thereto paraffin wax, a low-molecular weight polyolefin or the like. However, it has also been confirmed that the addition thereof in too small an amount does not exhibit any effect, while the addition thereof in too large an amount accelerates the deterioration of the developer.
As described in U.S. Pat. No. 3,590,000, a polyester resin is essentially so excellent in fixability that it can be sufficiently firmly fixed even by a non-contact fixing method. However, it is difficult to apply a heat roller fixing method to a polyester resin, because offset phenomenon frequently occurs. Japanese Patent Laid-Open Nos. 44836/1975, 37353/1982 and 109875/1982 disclose polyester resins which are prepared by using a polycarboxylic acid to thereby improve their offset resistance. However, some of these polyester resins do not exhibit sufficient offset resistance, while most of the others sacrifice their low-temperature fixability inherent in polyester, though they exhibit improved offset resistance, thus most of them being problematic. A toner containing a polyester resin tends to exhibit poorer fluidity than that of a toner containing a styrene resin. That is, the former tends to agglomerate to exhibit poor transportability in a developing device, so that its developability is lowered to give a low-quality visible image having unevenness, stain in non-image areas or the like.
For the purpose of improving the fluidity of a toner, a large amount of a fluidity improver such as hydrophobic silica powder is generally added. As a result of the addition thereof, however, when the surface of a latent image-supporting body is cleaned with a blade made of urethane rubber or the like, the toner particles get in between the blade and the support to result in incomplete cleaning, thus staining the visible image. Further, when an image-forming device of recycle type wherein a toner which has been developed, but has not been transferred is recycled to a developing device and re-used is employed, the hydrophobic silica fine powder is buried in the surface of the toner particle, so that the fluidity of the resulting toner lowers to give a low-quality visible image.
Further, a polyester itself has a proper frictional chargeability, so that a toner containing a polyester resin can be charged even without the addition of any charge controller. However, since the whole of a polyester resin is charged, the frictional charge of a toner containing a polyester resin gradually increases, while duplication is repeated. For example, after repeating duplication fifty thousand times or more, such a toner gives a visible image which is adversely affected by change in charge of the toner such as one having a reduced density.
The present invention has been made to solve the above mentioned problems and an object of the invention is to provide a developer which can be fixed according to the heat roller fixing method without causing offset even when no offset inhibitor is applied and is fixable at a low temperature.
Another object of the present invention is to provide a developer which exhibits an excellent fluidity and a prolonged life (slow deterioration) and does not cause blocking.